What is the starting dose of Triiodothyronine (T3) and what monitoring is required?

Starting Dose and Monitoring of Liothyronine (T3)

Starting Dose Recommendations

For mild hypothyroidism, start liothyronine at 25 mcg daily, which can then be increased by up to 25 mcg every 1-2 weeks until the usual maintenance dose of 25-75 mcg daily is reached. 1

Dose Adjustments Based on Clinical Scenario

For myxedema (severe hypothyroidism):

• Start at 5 mcg daily 1
• Increase by 5-10 mcg daily every 1-2 weeks 1
• Once 25 mcg daily is reached, increase by 5-25 mcg every 1-2 weeks 1
• Usual maintenance dose is 50-100 mcg daily 1

For elderly patients or those with cardiac disease:

• Start at 5 mcg daily with 5 mcg increments only at recommended intervals 1
• This conservative approach prevents cardiovascular complications, as T3 has more pronounced cardiovascular side effects compared to levothyroxine 1

For pediatric patients with congenital hypothyroidism:

• Start at 5 mcg daily with 5 mcg increments every 3-4 days 1
• Infants may require only 20 mcg daily for maintenance 1
• At 1 year, 50 mcg daily may be required 1
• Above 3 years, full adult dosage may be necessary 1

Critical Monitoring Parameters

Monitor TSH and free T4 levels every 6-8 weeks during dose titration, targeting TSH within the reference range (0.5-4.5 mIU/L) with normal free T4 levels. 2

Key Monitoring Considerations

T3 measurement is NOT useful for monitoring T3 replacement therapy:

• T3 levels show wide swings following liothyronine administration due to its rapid onset and dissipation 1
• Normal T3 levels can be seen even in over-replaced patients on levothyroxine, and this applies to T3 therapy as well 3
• Focus monitoring on TSH and free T4, not serum T3 2, 3

Once adequately treated, repeat testing every 6-12 months or if symptoms change. 2

Pharmacokinetic Considerations Affecting Monitoring

The rapid cutoff of liothyronine means metabolic effects persist for only a few days following discontinuance, unlike levothyroxine 1. This creates several monitoring challenges:

• Peak T3 levels occur approximately 4 hours after each dose 4
• T3 levels fluctuate significantly throughout the day with multiple daily dosing 4
• These fluctuations make single time-point T3 measurements unreliable for assessing adequacy of replacement 4

Special Monitoring Situations

For patients switching from levothyroxine to liothyronine:

• Discontinue levothyroxine and start liothyronine at a low dose 1
• Increase gradually according to patient response 1
• Remember that residual effects of levothyroxine may persist for the first several weeks of therapy 1

For patients with cardiac disease or atrial fibrillation:

• Consider repeating testing within 2 weeks of dose adjustment rather than waiting 6-8 weeks 2
• The wide swings in serum T3 levels and possibility of more pronounced cardiovascular side effects require closer monitoring 1

For patients on thyroid suppression therapy (75-100 mcg/day for 7 days):

• Administer cautiously in patients with suspected thyroid gland autonomy, as exogenous hormone effects will be additive to endogenous sources 1

Critical Pitfalls to Avoid

Never start thyroid hormone before ruling out adrenal insufficiency, as this can precipitate adrenal crisis—always initiate corticosteroids before T3 in suspected central hypothyroidism or hypophysitis 2

Avoid adjusting doses too frequently before reaching steady state—wait 6-8 weeks between adjustments to allow proper assessment 2

Do not rely on T3 levels to assess adequacy of replacement, as they bear little relation to thyroid status in patients on thyroid hormone replacement and normal levels can be seen in over-replaced patients 3

Monitor for signs of overtreatment, including TSH suppression below 0.1 mIU/L, which significantly increases risks for atrial fibrillation, osteoporosis, fractures, and cardiovascular complications 2